Carcass line with plural inflection points

ABSTRACT

In a pneumatic radial tire comprising a radial carcass and a belt, when the tire is mounted onto a normal rim and inflated under an internal pressure corresponding to 5% of a normal internal pressure, a carcass line of the carcass in radial section of the tire at a self-posture under no load has at least two inflection points between positions A and C and/or between positions C and B in which A is a position corresponding to each end of the belt at its maximum width, B is a position corresponding to a width of the normal rim and C is a position corresponding to a maximum width of the carcass.

This is a continuation of application Ser. No. 07/508,452 filed Apr. 13,1990 now U.S. Pat. No. 5,238,039.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to pneumatic radial tires having improved runningperformances.

Related Art Statement

Particularly, the improvement of the running performance in this type ofthe tire is strongly demanded in pneumatic radial tires for passengercars because it is required to always ensure a stable steering propertyin the passenger car in accordance with remarkable fullness of theequipment and performance in the passenger car as well as theenlargement and development of road network.

In order to improve the running performance of the pneumatic tire, it isnecessary to have sufficient rigidity in the tread portion and beadportion of the tire. For this purpose, there is considered acounter-measure of strengthening the reinforcement at the tread and beadportions, a countermeasure of raising an air pressure filled in theinside of the tire, and the like.

However, the quantity of reinforcing member required and used forstrengthening of the reinforcement is generally increased to undesirablyraise the cost. On the other hand, as the internal pressure becomeshigher, the ground contact area of the tread portion reduces to obstructthe transmission of necessary force, and consequently the steeringperformance, and traction and braking performances are degraded.

That is, the tire should be used under such an internal pressure as tohold a balance capable of accepting merits through the strengthening ofthe rigidity and demerits due to the degradation of the performances.However, the strengthening of the rigidity without causing the increaseof the cost can not be achieved by the above conventional technique.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a novelpneumatic radial tire for automobiles, particularly a passenger carwhich can advantageously realize the reinforcement of tread and beadportions in the tire without excessively using the reinforcing memberover the conventionally adopted limit and increasing the internalpressure.

According to the invention, there is the provision of a pneumatic radialtire having improved running performance comprising a carcass composedof at least one radial cord ply toroidally extending between a pair ofbead portions as a casing reinforcement for sidewall portions and treadportion connecting to the bead portions, at least one cord ply of whichcarcass being wound around a bead core of the bead portion from insideof the tire toward outside thereof to form a turnup structure, and abelt composed of plural cord layers arranged along a crown portion ofthe carcass as a tread reinforcement, characterized in that when thetire is mounted onto a normal rim and inflated under an internalpressure corresponding to 5% of a normal internal pressure, a carcassline of the carcass in radial section of the tire at a self-postureunder no load has at least two inflection points between positions A andC and/or between positions C and B in which A is a positioncorresponding to each end of the belt at its maximum width, B is aposition corresponding to a width of the normal rim and C is a positioncorresponding to a maximum width of the carcass. The inflection pointsdisappear when the tire is inflated to a normal internal pressure.

In a preferred embodiment of the invention, heights H₁, H₂ and M of thepositions A, B and C measured from a rim base line of the normal rim arewithin ranges of 0.80-1.0, 0.10-0.25 and 0.35-0.70 per a carcass maximumheight H measured from the same rim base line, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein:

FIG. 1 is a schematic view illustrating the carcass line according tothe invention;

FIG. 2 is a diagrammatically sectional view of the radial tire accordingto the invention;

FIG. 3 is a schematic view showing deformation behavior in the inflationunder a normal internal pressure;

FIG. 4 is a schematic view showing a comparison of carcass profile usedin the comparisons of FIGS. 5 and 6;

FIG. 5 is a graph showing a comparison of tension distribution in thebelt between the tire according to the invention and the conventionaltire; and

FIG. 6 is a graph showing a comparison of tension distribution in thecarcass ply between the tire according to the invention and theconventional tire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 is shown a left-half section of the pneumatic radial tire forpassenger car according to the invention in the radial direction thereofat a self-posture under no load when the tire is mounted onto a rim andinflated under an internal pressure corresponding to 5% of a normalinternal pressure. This tire is symmetry with respect to the equator ofthe tire.

In FIG. 1, numeral 1 is a bead portion, numeral 2 a sidewall portion,numeral 3 a tread portion, numeral 4 a bead core, numeral 5 a carcass,numeral 6 a belt and numeral 7 a normal rim closely fitted into the beadportion 1.

The carcass 5 is comprised of at least one radial cord ply toroidallyextending between the bead portions 1. In the illustrated embodiment,the carcass is comprised of a single cord ply, but two or three pliesmay be used. In any case, at least one cord ply is wound around the beadcore 4 from inside of the tire toward outside thereof to form a turnupstructure according to the usual manner.

In the above tire section at the self-posture under no load, anintersect point of a perpendicular line drawn from a widthwise end of abelt cord layer having a maximum width in the belt 6 toward a rim baseline RL of the normal rim 7 with the carcass 5 is defined as a positionA, and an intersect point of a perpendicular line drawn from the rimbase line RL with the carcass 5 at a position corresponding to an end ofthe rim width of the normal rim 7 is defined as a position B, and apoint corresponding to the maximum width of the carcass 5 is defined asa position C. Further, a straight line passing through the position Cand in parallel to the rim base line RL is CL, and a radius of an archaving a center on the straight line CL and passing through thepositions A and C is RA, and a radius of an arc having a center on thestraight line CL and passing through the positions B and C is RB.Moreover, in the carcass 5 shown by a meandering curve, a mark X is aninflection point of the carcass line and a mark Δ is a point mostseparated from a line segment connecting the position A or B to theinflection point X or a line segment connecting the adjoining inflectionpoints X to each other. Among arcs passing through the position A or B,the inflection point or adjoining inflection points and the point Δ, aradius of an arc having a center in the inner direction of the tire andlocating the point Δ toward the belt 6 with respect to the straight lineCL as a standard is represented by R_(i) ^(U) (i=1, . . . n^(U)), and aradius of an arc having a center in the inner direction of the tire andlocating the point Δ toward the normal rim 7 with respect to thestraight line CL as a standard is represented by R_(i) ^(L) (i= 1, . . .N^(L)), and a radius of an arc having a center in the outer direction ofthe tire and locating the point Δ toward the belt 6 with respect to thestraight line CL as a standard is represented by r_(i) ^(U) (i=1, . . .m^(U)), and a radius of an arc having a center in the outer direction ofthe tire and locating the point Δ toward the normal rim 7 with respectto the straight line CL as a standard is represented by r_(u) ^(L) (i=1,. . . m^(L)). According to the invention, the above radii are preferableto be within ranges of the following equations (1)-(4) in relation withthe radii RA and RB, respectively: ##EQU1##

As mentioned above, the carcass line is adopted to have at least twoinflection points each of between the positions A and C and between thepositions B and C, whereby the running performances of the tire can beimproved while using only the conventionally used members withoutrequiring new addition or change to the structure of the tire as well asthe reinforcing member for the tread and bead portions, so that theinvention has great merits in the quality and productivity of the tire.

The aforementioned peculiar profile of the carcass line is obtained bycontrolling a distribution of rubber gauge ranging from an inner wall ofa mold to the carcass ply in a rotational axis direction of the tire inthe building-up and vulcanization of the tire.

Moreover, if it is intended to strictly control the change or scatteringof the carcass line due to a so-called "rubber flow" in thevulcanization, the following method is particularly effective.

That is, the cord ply of the carcass 5 extending between the bead cores4 is merely arranged along the inner shape of the tire according to theconventional building-up and vulcanization method. On the other hand,according to the invention, an inner liner 8 having locally differentthickness portions 8' may be used as shown in FIG. 2.

In addition, the change in the curvature of the carcass line mayproperly be controlled by forming continuous protrusions on a peripheryof a bladder for vulcanization at positions corresponding to curvaturechanged portions of the carcass line.

By considering such a laborless means, the carcass line according to theinvention can properly be controlled without causing the variation orscattering in the productivity as previously explained.

According to the invention, the carcass line of the pneumatic radialtire in the radial section at the self-posture under an internalpressure corresponding to 5% of a normal internal pressure has at leasttwo inflection points between the positions A and C and/or between thepositions B and C.

In FIG. 3 is shown carcass profiles having two inflection points each ofbetween the positions A and C and between the positions B and C beforeand after the inflation under a normal internal pressure, in which asolid line is a carcass profile before the inflation and dotted linesare a carcass profile after the inflation. As illustrated in the dottedline profile, the inflection points disappear when the tire is inflatedto a normal inflation pressure. As to the deformation between thepositions A and C, a portion of the carcass line having a center ofcurvature in an inside direction of the tire in adjacent to the positionA deforms inward the tire without changing the curvature center, while aportion of the carcass line having a center of curvature in an outsidedirection of the tire deforms outward the tire with changing thecurvature center into the inside of the tire.

As to the deformation between the positions B and C, a portion of thecarcass line having a center of curvature in an outside direction of thetire beneath the position C deforms outward the tire with changing thecurvature center into the inside of the tire, while a portion of thecarcass line having a center of curvature in an inside direction of thetire in adjacent to the position A deforms inward the tire withoutchanging the curvature center.

The strain produced along the carcass line due to the deformation of thetire is considered as follows.

In the portion wherein the curvature center changes from the outside ofthe tire toward the inside thereof, the carcass line has room againstthe deformation of the tire before the inflation under normal internalpressure, so that the strain produced along the carcass line after theinflation under normal internal pressure becomes small, and consequentlythe tension is small. Similarly, when the curvature center of thecarcass line is held inside the tire before and after the inflation, ifthe carcass line is deformed inward during the inflation under normalinternal pressure, the tension is small due to the presence of roomagainst the deformation, while if the carcass line is deformed outwardduring inflation under normal internal pressure, the tension becomeslarge to a certain extent.

In the tension distribution between the positions A and C, the carcassline adjoining to the position A deforms inward with holding thecurvature center at the inside of the tire through the inflation undernormal internal pressure, so that the tension is small. Further, theposition of curvature center in the carcass line located beneath theabove carcass line portion changes from the outside of the tire towardthe inside thereof, so that the tension is small.

That is, since the carcass tension near the widthwise end of the belt 6is small, the belt tension increases by a quantity that the carcasstension is reduced from a balance between the internal pressure and thesum of carcass tension and belt tension.

In the tension distribution between the positions B and C, the curvaturecenter in the carcass line located beneath the position C changes fromthe outside of the tire toward the inside thereof through the inflationunder normal internal pressure, so that the tension is small. On theother hand, the carcass line located beneath the above carcass lineportion deforms outward while holding the curvature center at the insideof the tire, so that the tension is not small. Therefore, a portion ofthe carcass tension reduced beneath the position C shifts to the lowerportion of the carcass line from a balance between the internal pressurein the bead portion and the carcass tension to further enhance thetension near to the bead portion.

As a result, the tension is ununiformly distributed through the carcassline, whereby a large tension can be given to the tread portionrequiring a large road gripping force and to the bead portion requiringa large transmission force to the rim. Consequently, the various runningperformances are considerably improved.

Moreover, in order to give the most effective tension to the tread andbead portions, the adequate arrangement of radius of curvature of thecarcass line and position of inflection point or radii R_(i) ^(U) (i=1,. . . n^(U)), R_(i) ^(L) (i=1 , . . . n^(L)), r_(i) ^(U) (i=1, . . .m^(U)), and r_(i) ^(L) (i=1, . . . m^(L)), per RA and RB as shown inFIG. 1 are favorable to be within the following ranges, respectively:##EQU2##

Further, when the thickness of the inner liner 8 is locally changed aspreviously shown in FIG. 2, it is desirable that the portion 8' of FIG.2 is 1.7 to 8 times, preferably 2 to 4 times higher than the averagethickness of the inner liner in order to sufficiently suppress thechange of the casing line.

The following example is given in illustration of the invention and isnot intended as a limitation thereof.

There was provided a passenger car tire (size: 165 SR13) comprising twosteel cord belt layers and one carcass cord ply as shown in FIGS. 1 and2 and having a carcass line satisfying R₁ ^(U) /RA=0.3, r₁ ^(U) /RA=0.9,R₁ ^(L) /RB=0.3, r₁ ^(L) /RB=0.6, and R₂ ^(L) /RB=0.55. Then, an actualrunning test was made with respect to this tire together with theconventional tire having a carcass profile of natural equilibrium state.As a result, when the running stability was evaluated by the 10 pointmethod, it was improved by 1.5 points in the tire according to theinvention as compared with the conventional tire.

The profile of the carcass line in the tire according to the inventionand the conventional tire is shown in FIG. 4, in which a solid line is anatural equilibrium profile of the conventional tire before theinflation under normal internal pressure and dotted lines are thecarcass profile of the invention before the inflation under normalinternal pressure. Furthermore, the distribution of belt tension afterthe inflation under normal internal pressure in the tire according tothe invention and the conventional tire is shown in FIG. 5, while thedistribution of carcass ply tension after the inflation under normalinternal pressure in the tire according to the invention and theconventional tire is shown in FIG. 6.

As mentioned above, according to the invention, the belt rigidity andthe bead rigidity of the tire can advantageously be increased withoutenhancing the belt and bead reinforcements or increasing the internalpressure, which contributes to the improvement of the runningperformances in the tire.

What is claimed is:
 1. A pneumatic radial tire having improved running performance comprising: a carcass composed of at least one radial cord ply toroidally extending between a pair of bead portions as a casing reinforcement for sidewall portions and tread portion connecting to the bead portions, at least one cord ply of said carcass being wound around a bead core of the bead portion from inside of the tire toward outside thereof to form a turnup structure, and a belt composed of plural cord layers arranged along a crown portion of the carcass as a tread reinforcement, wherein, when the tire is mounted onto a normal rim and inflated under an internal pressure corresponding to 5% of a normal internal pressure, a carcass line of the carcass in radial section of the tire at a self-posture under no load has at least two inflection points in a first zone on a side of carcass with respect to an equatorial line between positions A and C and/or two inflection points in a second zone on said side of said carcass between positions C and B, in which position A is a point on said carcass line corresponding to each end of the belt at its maximum width defined by a line extending from a widthwise end of said belt perpendicular to a rim base line to said carcass line, B is a point on said carcass line corresponding to a width of the normal rim defined by a line extending from an end of said rim perpendicular to said rim base line to said carcass line and C is a point on said carcass line corresponding to a maximum width of the carcass, and wherein heights H₁, H₂ and M of the positions A, B and C measured from said rim base line of the normal rim are respectively within ranges of 0.80-1.0, 0.10-0.25 and 0.35-0.70 of a carcass maximum height H measured from said same rim base line.
 2. A pneumatic radial tire having improved running performance comprising: a carcass composed of at least one radial cord ply toroidally extending between a pair of bead portions as a casing reinforcement for sidewall portions and tread portion connecting to the bead portions, at least one cord ply of said carcass being wound around a bead core of the bead portion from inside of the tire toward outside thereof to form a turnup structure, and a belt composed of plural cord layers arranged along a crown portion of the carcass as a tread reinforcement, wherein, when the tire is mounted onto a normal rim and inflated under an internal pressure corresponding to 5% of a normal internal pressure, a carcass line of the carcass in radial section of the tire at a self-posture under no load has at least two inflection points in a first zone on a side of carcass with respect to an equatorial line between positions A and C and/or two inflection points in a second zone on said side of said carcass between positions C and B, in which position A is a point on said carcass line corresponding to each end of the belt at its maximum width defined by a line extending from a widthwise end of said belt perpendicular to a rim base line to said carcass line, B is a point on said carcass line corresponding to a width of the normal rim defined by a line extending from an end of said rim perpendicular to said rim base line to said carcass line and C is a point on said carcass line corresponding to a maximum width of the carcass, and wherein said inflection points disappear when the tire is inflated to a normal internal pressure. 